1. Field of the Invention
The invention relates to two piece molded tape cassettes, methods of manufacturing two piece molded tape cassettes and a spring member contained in a cassette for pressing a tape against a magnetic head.
2. Description of the Related Art
Tape cassettes shells are generally formed from two sections that are mated together to enclose a tape. The tape is wound around two spools that are mounted in the shell, and a portion of the tape between the spools extends across a head insertion area. Generally the head insertion area is defined by a bracket that is integrally formed with one or both sections of the cassette shell. In addition, azimuth posts are usually formed on either side of the head insertion area to help guide the tape through the head insertion area. The azimuth posts are often connected to, or integrally formed with the bracket, or with support ribs that are intended to support the azimuth posts in an upright position with respect to the floor of the cassette.
FIG. 1 shows a portion of a base section of a prior art tape cassette that includes a bracket 22 defining a head insertion area, azimuth posts 24, front support ribs 28 and rear support ribs 26. The front and rear support ribs 28, 26 are integrally formed with the bracket 22 and the azimuth posts 24.
As shown in FIG. 5, a spring member 30, with a contact pad 34 is usually mounted on the rear support ribs 26, just behind the azimuth posts 24. The portion of the tape between the spools passes over the front support ribs 28, in front of the azimuth posts 24. When a magnetic head is inserted into the head insertion area, the contact pad 24 of the spring member 30 pushes the tape against the magnetic head.
A typical prior art spring member 30 is shown in FIG. 3. The spring member 30 is comprised of a main body portion 32, a contact pad 34 mounted on the main body portion 32, and end portions 36. The spring member 30 is typically formed from a flat piece of thin metal, and the ends 36 are usually bent at approximately a 90.degree. angle to the main body portion 32.
Because the tape passing in front of the azimuth posts must pass over the top surface of the front support ribs 28, and because the contact pad 34 must be centered on the tape to press the entire height of the tape against a magnetic head inserted into the head insertion area, the rear support ribs 26 are formed to have a top surface that is higher off the floor 20 of the base section than the top surface of the front support ribs 28. The higher height of the rear support ribs 26 is necessary to ensure that the contact pad 34 is centered on the tape.
The two sections of the cassette shell are generally formed by injection molding. During molding, a heated liquid resin material is injected into a steel mold under pressure. The molds are often "family molds" that have at least one cavity for forming a base section, and at least one cavity for forming a cover section. After the cavities in the mold are filled with liquid resin, the mold is cooled until the resin solidifies. The solidified resin material is then ejected from the mold, and cooled to room temperature.
The rear support ribs 26 have a greater height, and a greater mass than the front support ribs 28. During cooling of the cassette shell sections the resin material contracts. The contraction of a support rib applies a stress on an azimuth post that tends to pull the azimuth posts toward the support rib. Because the rear support ribs 26 are higher, and have a greater mass, than the front support ribs 28, the stress applied to azimuth posts 24 by the support ribs is unequal. The azimuth posts tend to be pulled backwards toward the rear support ribs 26. As a result, the azimuth posts 24 are not perpendicular to the floor 20, after cooling.
For proper operation of the tape cassette, it is important that the azimuth posts 24 be as perpendicular as possible. Because of the unequal height support ribs, however, the azimuth posts 24 are not perpendicular to the base 20 of the cassette shell.